This invention relates to an ultrasonic wave tomographic imaging system which radiates an ultrasonic wave to an object and reproduces an internal condition of said object as an image from the reflected the wave, the transmitted wave, refracted wave or the scattered wave received from said object, particularly to an ultrasonic wave tomographic imaging system which provides an internal acoustic image having a high S/N ratio.
An ultrasonic wave tomographic imaging system radiates an ultrasonic wave to an object of which the internal condition is to be observed, receives the reflected wave, the transmitted wave or the scattered wave returning from the inside of said object as an internal acoustic image of said object and reproduces an internal condition of said object on the basis of said received waves.
As a system for observing internal condition, an X-ray diagnostic system is widely used. An ultrasonic wave tomographic imaging system, as compared with such an X-ray diagnostic system, is especially not destructive for organs and is less dangerous in the case of a human body as an object. Moreover, it has the merit that it is suited for diagnosis of the soft organs of human body.
As an ultrasonic wave tomographic imaging system, the camera system utilizing an ultrasonic wave lens is already known. This technique is disclosed in the U.S. Pat. No. 3,937,066 "Ultrasonic Camera System and Method".
This ultrasonic camera system has the merits that the desired region can be examined on a real time basis movement of the image can be observed, and the system is superior to the other ultrasonic wave tomographic imaging systems (such as the pulse echo system).
As indicated in FIG. 1, this prior ultrasonic wave tomographic imaging system using a camera is composed of the ultrasonic wave generating means (a generator 1) and the ultrasonic wave receiving means (a receiver 2) which are respectively provided on the opposite side of the object 3.
The generator 1 comprises the ultrasonic wave generator 12 consisting of an electric-acoustic transducer such as a crystal, etc. housed in the case 11, and the contact surface 13 of the case 11 to the object is composed of a flexible organic film having an acoustic impedance which is almost equal to that of the object.
The receiver 2 comprises an ultrasonic wave lens 22 which functions as the ultrasonic wave optical system and an acoustic transducer 23 housed in the case 21, and the contact surface 24 of the case 21 is also composed of an organic film as in the case of said contact surface 13.
The cases 11 and 21 are filled with a medium (for example, water) which has an acoustic impedance almost equal to that of the object 3 such as a human body 3.
The generator 1 and receiver 2 composed respectively as explained above are provided in contact with the object 3 as indicated in FIG. 1, and the ultrasonic wave generator 12 radiates an ultrasonic wave to the object 3.
An acoustic image of object 3 by an ultrasonic wave is focused on the acoustic transducer 23 by means of the ultrasonic wave lens 22.
The ultrasonic wave lens 22 converges the ultrasonic waves as is well known and focuses an acoustic image at the section X in the location determined by the focal distance of the ultrasonic wave lens 22 and a distance between the ultrasonic wave lens 22 and the acoustic transducer 23 thereon.
As the acoustic transducer 23, an acoustic-visual image converter which utilizes an aluminum suspension liquid or an acoustic-electric transducer based on the piezoelectric effect can be used.
In the case of the ultrasonic wave tomographic imaging system of this type, an ultrasonic image of the imaging plane is correctly focused on the transducer 23 in the ideal case, but actually an ultrasonic wave image of the plane X is reflected, refracted or scattered until it reaches the surface of transducer 23, and moreover these images are superimposed to form an obscured image, or said ultrasonic wave image is degraded due to the following major causes of noise, namely the space noise wherein images of other planes than X plane are superimposed to form an obscured image and the timing noise wherein images are superimposed on the image of the plane X with some delay because of the many reflections, refractions and scatterings before they reach the transducer 23 from the generator 12.
FIG. 2 explains the principle why such space noises are superimposed. The ultrasonic waves that are reflected, refracted or scattered (hereinafter simply referred to as reflected) from the point S on the plane X form an image SA having an obscured space intensity distribution as a result of reflections during the travelling process for focusing at the point S' on the surface Y of transducer 23.
Similarly, the ultrasonic waves reflected from the point S.sub.3 on the plane X also form a image SD having the obscured space intensity distribution. In addition, the ultrasonic waves reflected from the point S.sub.1 which is nearer to the lens 22 than the plane X are focused to the point S'.sub.1 which is further than the surface Y, but form a image SB having the unfocused space intensity distribution at the surface Y.
On the other hand, the ultrasonic waves reflected from the point S.sub.2 which is further from the lens 22 than the plane X are focused at the point S'.sub.2 which is located before the surface Y and thereby form a image SC having the diverged space intensity distribution at the surface Y. As a result, it is a problem to be solved in the ultrasonic wave tomographic imaging system of this type that mutually focused intensity distributions are superimposed as the space noises of other images and thereby an image is obscured.
FIGS. 3(A) and (B) explain the principle where the timing noises are superimposed. The ultrasonic waves reflected from the point S" on the plane X reach the point S"' on the surface Y within a specified time .DELTA.T. On the other hand, the ultrasonic waves reflected from the point S' on the plane X are also multiply reflected at the reflection surfaces Z.sub.1, Z.sub.2 and reach the point S"' on the surface Y after a delay of .DELTA.T via the point S". It is another problem to be solved in the ultrasonic wave tomographic imaging system of this type that the pictures explained above are superimposed as in the case of FIG. 3 (B), the illustrated intensity distribution image S'A on the time axis and thereby, the ultrasonic waves reflected from the point S' becomes the timing noise, obscuring the image.